NO.sub.x emissions from gas flames can be created either through the Zeldevitch mechanism (often called thermal NO.sub.x) or through the formation of HCN and/or NH.sub.3 which can then be ultimately oxidized to NO.sub.x (prompt NO.sub.x). Thermodynamic calculations typically show that NO.sub.x emissions measured from natural gas flames are well below, one to two orders of magnitude, the thermodynamic equilibrium value. This indicates that in most situations NO.sub.x formation is kinetically controlled. Kinetic calculations indicate that thermal NO.sub.x emissions are typically the most important source of NO.sub.x for natural gas flames, with the NO.sub.x being created through the following reactions: EQU N+O.sub.2 =NO+O (1) EQU N+OH=NO+H (2) EQU N.sub.2 +O=NO+N (3)
Kinetic calculations were performed using a PC version of the CHEMKIN computer program. Calculations using this program have provided valuable insight into changes in the burner fuel and air mixing characteristics which can lower NO.sub.x emissions.
As the name implies, thermal NO.sub.x can be controlled by regulation of the peak flame temperature, and as shown in FIG. 1 using kinetic calculations, if the temperature can be lowered enough the NO.sub.x emissions from a "true" premixed natural gas flame operating at 15% excess air can be reduced to extremely low values (less than 1 ppmv). In effect FIG. 1 shows the relationship between thermal NO.sub.x and temperature since for a premixed natural gas flame with an excess of oxygen, thermal NO.sub.x is the only route by which any significant NO.sub.x emissions are created.
Under appropriate flame conditions the formation of prompt NO.sub.x can also be important when burning natural gas. The kinetic model used shows that under fuel rich conditions, particularly when the stoichiometry is under about 0.6, both HCN and NH.sub.3 can be formed through reaction of CH with N.sub.2 to form HCN and N. These calculations were conducted using gas and air mixtures with stoichiometries ranging from 1.0 to 0.4. The model predicts that prompt NO.sub.x becomes important at higher stoichiometries when the temperature is lower; see FIG. 2. Below a stoichiometry of 0.5 almost all the NO.sub.x formed is prompt NO.sub.x. The rate of prompt NO.sub.x formation (as the name implies) is also very rapid, being nearly complete in about 1 millisecond at a temperature of 2400.degree. F.
Kinetic calculations also indicate that hydrocarbon fragments, in addition to being important for prompt NO.sub.x, are also important for thermal NO.sub.x formation since they can act as a source of O atoms and OH radicals. Kinetic calculations show the importance of the hydrocarbon concentration in the formation of NO.sub.x, even under oxidizing conditions. At a temperature of 3400 .degree. F. the predicted NO.sub.x emissions were about 4 ppmv after 5 ms residence time for a mixture of N.sub.2, O.sub.2, H.sub.2 O, and CO.sub.2 when hydrocarbons were not present, as compared to 80 ppmv when combustion of about 1% CH.sub.4 was present in the gas mixture. If the concentration of methane initially present was reduced to about 0.5%, the NO.sub.x concentration after 5 ms was reduced to about 75 ppmv. The kinetic model used predicts that the following mechanisms are important:
1. Reaction of CH.sub.4 with O.sub.2, OH and H to form CH.sub.3 PA1 2. Reaction of CH.sub.3 with O.sub.2 to form CH.sub.3 O and O PA1 3. Reaction of N.sub.2 with O to form NO and O PA1 4. Various reactions to form OH PA1 5. Reaction of N.sub.2 with OH to form NO and NH
Low NO.sub.x gas burners have been undergoing considerable development in recent years as governmental regulations have required burner manufacturers to comply with lower and lower NO.sub.x limits. Most of the existing low NO.sub.x gas burner designs are nozzle mix designs. In this approach the fuel is mixed with the air immediately downstream of the burner throat. These designs attempt to reduce NO.sub.x emissions by delaying the fuel and air mixing through some form of either air staging or fuel staging combined with flue gas recirculation ("FGR"). Delayed mixing can be effective in reducing both flame temperature and oxygen availability and consequently in providing a degree of thermal NO.sub.x control. However, delayed mixing burners are not effective in reducing prompt NO.sub.x emissions and can actually exacerbate prompt NO.sub.x emissions. Delayed mixing burners can also lead to increased emissions of CO and total hydrocarbons. Stability problems often exist with delayed mixing burners which limit the amount of FGR which can be injected into the flame zone. Typical FGR levels at which current burners operate are at a ratio of around 20% recirculated flue gas relative to the total stack gas flow.
A further type of low NO.sub.x burner which has been developed in recent years is the premixed type burner. In this approach, the fuel gas and oxidant gases are mixed well upstream of the burner throat, e.g. at or prior to the windbox. These burners can be effective in reducing both thermal and prompt NO.sub.x emissions. However, problems with premixed type burners include difficulty in applying high air preheat, concerns about flashback and explosions, and difficulties in applying the concept to duel fuel burners. Premix burners also typically have stability problems at high FGR rates.